US12506114B2

US12506114B2 - Directly bonded metal structures having aluminum features and methods of preparing same

Info

Publication number: US12506114B2
Application number: US18/148,332
Authority: US
Inventors: Cyprian Emeka Uzoh
Original assignee: Adeia Semiconductor Bonding Technologies Inc
Current assignee: Adeia Semiconductor Bonding Technologies Inc
Priority date: 2022-12-29
Filing date: 2022-12-29
Publication date: 2025-12-23
Also published as: TW202429589A; WO2024145034A1; CN120604336A; US20240222315A1; EP4643383A1; KR20250130627A; JP2025542482A

Abstract

An element, a bonded structure including the element, and a method of forming the same are disclosed. The bonded structure can include a first element having a first nonconductive field region and a first conductive feature. A surface of the first nonconductive field region and a surface of the first conductive feature at least partially defining a bonding surface of the first element. The first conductive feature includes a first portion and a second portion over the first portion and at least partially defines the surface of the first conductive feature. The first portion includes aluminum. The first conductive feature has a continuous sidewall along the first portion and the second portion. The second portion includes different metal composition from the first portion or comprising fluorine at the surface of the first conductive feature. The bonded structure can include a second element having a second nonconductive field region and a second conductive feature. A surface of the second nonconductive field region is directly bonded to the first nonconductive field region without an intervening adhesive along a bond interface and a surface of the second conductive feature is directly bonded to the second conductive feature without an intervening adhesive along the bond interface.

Description

BACKGROUND Field

The field relates to bonded structures and methods of forming direct metal bonds that include aluminum features.

Description of the Related Art

Microelectronic elements, such as integrated device dies or chips, may be mounted or stacked on other elements thereby forming a bonded structure. Direct metal bonding can be conducted at low temperatures and without external pressure. For example, direct hybrid bonding involves directly bonding non-conductive features (e.g., inorganic dielectrics) of different elements together, without intervening adhesives, while also directly bonding conductive features (e.g., metal pads or lines) of the elements together. For example, a microelectronic element can be mounted to a carrier, such as an interposer, a reconstituted wafer or element, etc. As another example, a microelectronic element can be stacked on top of another microelectronic element, e.g., a first integrated device die can be stacked on a second integrated device die. Each of the microelectronic elements can have conductive pads for mechanically and electrically bonding the elements to one another. There is a continuing need for improved methods for forming the bonded structure.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific implementations will now be described with reference to the following drawings, which are provided by way of example, and not limitation.

FIG. 1A is a schematic cross-sectional side view of two elements prior to direct hybrid bonding.

FIG. 1B is a schematic cross-sectional side view of the two elements shown in FIG. 1A after direct hybrid bonding.

FIG. 2 is a schematic cross-sectional side view of a bonded structure including elements bonded through solder balls.

FIGS. 3A-3E are schematic cross sections showing a method of forming copper pads over aluminum interconnects formed in a back-end-of-line (BEOL) layer of an element.

FIGS. 4A-4E are schematic cross sections showing a method of forming aluminum pads over aluminum interconnects formed in a BEOL layer of an element.

FIGS. 5A to 5F are schematic cross sections showing a method of forming a bonding surface of an element that includes conductive features, according to an embodiment.

FIG. 5G is a schematic cross-sectional side view of a bonded structure that includes the element formed in FIG. 5F and a second element (such as a wafer).

FIG. 5H is a schematic cross-sectional side view of a bonded structure that includes the element formed in FIG. 5F and a plurality of second elements (such as dies).

FIGS. 6A to 6E are schematic cross sections showing a method of forming a bonding surface of an element that includes conductive features having fine grain aluminum, according to an embodiment.

FIG. 6F is a schematic cross-sectional side view of a bonded structure that includes the element formed in FIG. 6E prior to annealing.

FIG. 6G is a schematic cross-sectional side view of a bonded structure that includes the element formed in FIG. 6F after annealing.

FIG. 6H is a schematic cross-sectional side view of a bonded structure that includes the element formed in FIG. 6F or 6G after singulation.

FIG. 6I is a schematic cross-sectional side view of the element of FIG. 6D or 6E after forming a protective layer and singulation.

FIG. 6J is a schematic cross-sectional side view of the singulated elements of FIG. 6I after removing the protective layer.

FIG. 6K shows the element of FIG. 6D after singulation and during a termination process.

FIG. 6L is a schematic cross-sectional side view of a bonded structure that includes the singulated elements of FIG. 6K.

FIGS. 7A to 7E are schematic cross sections showing a method of forming a bonding surface of an element that includes conductive features having fine grain copper, according to an embodiment.

FIG. 7F is a schematic cross-sectional side view of a bonded structure that includes the element formed in FIG. 7E.

DETAILED DESCRIPTION

Various embodiments disclosed herein relate to directly bonded structures in which two or more elements can be directly bonded to one another without an intervening adhesive. FIGS. 1A and 1B schematically illustrate a process for forming a directly hybrid bonded structure without an intervening adhesive according to some embodiments. In FIGS. 1A and 1B, a bonded structure 100 comprises first and second elements 102 and 104 that can be directly bonded to one another at a bond interface 118 without an intervening adhesive. Two or more microelectronic elements 102 and 104 (such as semiconductor elements, including, for example, integrated device dies, wafers, passive devices, individual active devices such as power switches, etc.) may be stacked on or bonded to one another to form the bonded structure 100. Conductive features 106 a (e.g., contact pads, exposed ends of vias or through substrate vias (TSVs), elongated traces, etc. of the first element 102 may be mechanically and electrically connected to corresponding conductive features 106 b of the second element 104. Any suitable number of elements can be stacked in the bonded structure 100. For example, a third element (not shown) can be stacked on the second element 104, a fourth element (not shown) can be stacked on the third element, and so forth. Additionally or alternatively, one or more additional elements (not shown) can be stacked laterally adjacent one another along the first element 102. In some embodiments, the laterally adjacent additional stacked element(s) may be smaller than the second element. In some embodiments, the laterally adjacent additional stacked element(s) may be less than half the size of the second element in lateral dimensions.

In some embodiments, the elements 102 and 104 are directly bonded to one another without an adhesive. In various embodiments, a non-conductive field region, which includes a non-conductive or dielectric material can serve as a first bonding layer 108 a of the first element 102. The first bonding layer 108 a can be directly bonded to a corresponding non-conductive field region that includes a non-conductive or dielectric material serving as a second bonding layer 108 b of the second element 104 without an adhesive. The non-conductive bonding layers 108 a and 108 b can be disposed on respective front sides 114 a and 114 b of device portions 110 a and 110 b, such as a semiconductor (e.g., silicon) portion of the elements 102, 104. Active devices and/or circuitry can be patterned and/or otherwise disposed in or on the device portions 110 a and 110 b. Active devices and/or circuitry can be disposed at or near the front sides 114 a and 114 b of the device portions 110 a and 110 b, and/or at or near opposite backsides 116 a and 116 b of the device portions 110 a and 110 b. Bonding layers can be provided on front sides and/or back sides of the elements. The non-conductive material can be referred to as a non-conductive bonding region or bonding layer 108 a of the first element 102. In some embodiments, the non-conductive bonding layer 108 a of the first element 102 can be directly bonded to the corresponding non-conductive bonding layer 108 b of the second element 104 using dielectric-to-dielectric bonding techniques. For example, non-conductive or dielectric-to-dielectric bonds may be formed without an adhesive using the direct bonding techniques disclosed at least in U.S. Pat. Nos. 9,564,414; 9,391,143; and 10,434,749, the entire contents of each of which are incorporated by reference herein in their entirety and for all purposes. It should be appreciated that in various embodiments, the bonding layers 108 a and/or 108 b can comprise a non-conductive material such as a dielectric material, for example, silicon oxide, or an undoped semiconductor material, for example, undoped silicon. Suitable dielectric bonding surface or materials for direct bonding include but are not limited to inorganic dielectrics, such as silicon oxide, silicon nitride, or silicon oxynitride, and can include carbon, such as silicon carbide, silicon oxycarbonitride, low K dielectric materials, SiCOH dielectrics, silicon carbonitride, glass, ceramics, glass-ceramics, or diamond-like carbon or a material comprising a diamond surface. Such carbon-containing materials can be considered inorganic, despite the inclusion of carbon. In some embodiments, the dielectric materials do not comprise adhesive or polymer materials, such as epoxy, resin or molding materials. In some embodiments, including embodiments described hereinbelow, the dielectric bonding surfaces are defined by wafer-level processing of the underlying devices, such as the upper interlevel dielectric or passivation layers formed in back-end-of-line (BEOL) processing of an integrated circuit, and no separate bonding layer is deposited after formation of the underlying device.

In some embodiments, the device portions 110 a and 110 b can have significantly different coefficients of thermal expansion (CTEs) defining a heterogenous structure. The CTE difference between the device portions 110 a and 110 b, and particularly between bulk semiconductor, typically single crystal portions of the device portions 110 a, 110 b, can be greater than 5 ppm or greater than 10 ppm. For example, the CTE difference between the device portions 110 a and 110 b can be in a range of 5 ppm to 100 ppm, 5 ppm to 40 ppm, 10 ppm to 100 ppm, or 10 ppm to 40 ppm. In some embodiments, one of the device portions 110 a and 110 b can comprise optoelectronic single crystal materials, including perovskite materials, that are useful for optical piezoelectric or pyroelectric applications, and the other of the device portions 110 a, 110 b comprises a more conventional substrate material (Si, Ge, SiGe, III-V material, etc.). For example, one of the device portions 110 a, 110 b comprises lithium tantalate (LiTaO₃) or lithium niobate (LiNbO₃), and the other one of the device portions 110 a, 110 b comprises silicon (Si), quartz, fused silica glass, sapphire, or a glass. In other embodiments, one of the device portions 110 a and 110 b comprises a III-V single semiconductor material, such as gallium arsenide (GaAs) or gallium nitride (GaN), and the other one of the device portions 110 a and 110 b can comprise a non-III-V semiconductor material, such as silicon (Si) and/or germanium (Ge), or can comprise other materials with similar CTE, such as quartz, fused silica glass, sapphire, or a glass.

In various embodiments, direct hybrid bonds can be formed without an intervening adhesive. For example, nonconductive bonding surfaces 112 a and 112 b can be polished to a high degree of smoothness. The nonconductive bonding surfaces 112 a and 112 b can be polished using, for example, chemical mechanical polishing (CMP). The roughness of the polished bonding surfaces 112 a and 112 b can be less than 30 Å rms. For example, the roughness of the bonding surfaces 112 a and 112 b can be in a range of about 0.1 Å rms to 15 Å rms, 0.5 Å rms to 10 Å rms, or 1 Å rms to 5 Å rms. The bonding surfaces 112 a and 112 b can be cleaned and exposed to a plasma and/or etchants to activate the surfaces 112 a and 112 b. In some embodiments, the surfaces 112 a and 112 b can be terminated with a species after activation or during activation (e.g., during the plasma and/or etch processes). Without being limited by theory, in some embodiments, the activation process can be performed to break chemical bonds at the bonding surfaces 112 a and 112 b, and the termination process can provide additional chemical species at the bonding surfaces 112 a and 112 b that improves the bonding energy during direct bonding. In some embodiments, the activation and termination are provided in the same step, e.g., a plasma to activate and terminate the surfaces 112 a and 112 b. In other embodiments, the bonding surfaces 112 a and 112 b can be terminated in a separate treatment to provide the additional species for direct bonding. In various embodiments, the terminating species can comprise nitrogen. For example, in some embodiments, the bonding surface(s) 112 a, 112 b can be exposed to a nitrogen-containing plasma. Further, in some embodiments, including embodiments described in more detail hereinbelow, the bonding surfaces 112 a and 112 b can be exposed to fluorine. For example, there may be one or multiple fluorine peaks at or near a bond interface 118 between the first and second elements 102, 104. Thus, in the directly bonded structure 100, the bond interface 118 between two non-conductive materials (e.g., the bonding layers 108 a and 108 b) can comprise a very smooth interface with higher nitrogen content and/or fluorine peaks at the bond interface 118. In embodiments described hereinbelow, fluorine can be found at or near the bond interface 118 of both non-conductive regions 108 a/108 b and conductive regions 106 a/106 b of the bonded structure 100. Additional examples of activation and/or termination treatments may be found throughout U.S. Pat. Nos. 9,564,414; 9,391,143; and 10,434,749, the entire contents of each of which are incorporated by reference herein in their entirety and for all purposes. The roughness of the polished bonding surfaces 112 a and 112 b can be slightly rougher (e.g., about 1 Å rms to 30 Å rms, 3 Å rms to 20 Å rms, or possibly rougher) after an activation process.

In various embodiments, conductive features 106 a of the first element 102 can also be directly bonded to corresponding conductive features 106 b of the second element 104. For example, a direct hybrid bonding technique can be used to provide conductor-to-conductor direct bonds along the bond interface 118 that includes covalently direct bonded non-conductive-to-non-conductive (e.g., dielectric-to-dielectric) surfaces, prepared as described above. In various embodiments, the conductor-to-conductor (e.g., conductive feature 106 a to conductive feature 106 b) direct bonds and the dielectric-to-dielectric hybrid bonds can be formed using the direct bonding techniques similar to those disclosed at least in U.S. Pat. Nos. 9,716,033 and 9,852,988, the entire contents of each of which are incorporated by reference herein in their entirety and for all purposes. In embodiments described hereinbelow, aluminum can be employed in place of copper at the bonding surfaces, and fluorine termination can be employed in place of the nitrogen terminations described in the incorporated disclosures. In direct hybrid bonding embodiments described herein, conductive features are provided within non-conductive bonding layers, and both conductive and nonconductive features are prepared for direct bonding, such as by the planarization, activation and/or termination treatments described above. Thus, the bonding surface prepared for direct bonding includes both conductive and non-conductive features. Specific additional preparation options for directly bonding aluminum conductive features are described hereinbelow.

For example, non-conductive (e.g., dielectric) bonding surfaces 112 a, 112 b (for example, inorganic dielectric surfaces) can be prepared and directly bonded to one another without an intervening adhesive as explained above. Conductive contact features (e.g., conductive features 106 a and 106 b which may be partially or fully surrounded by non-conductive dielectric field regions within the bonding layers 108 a, 108 b) may also directly bond to one another without an intervening adhesive. In various embodiments, the conductive features 106 a, 106 b can comprise discrete pads or traces at least partially embedded in the non-conductive field regions. In some embodiments, the conductive contact features can comprise exposed contact surfaces of through substrate vias (e.g., through silicon vias (TSVs)). In some embodiments, the respective conductive features 106 a and 106 b can be recessed below exterior (e.g., upper) surfaces (non-conductive bonding surfaces 112 a and 112 b) of the dielectric field region or non-conductive bonding layers 108 a and 108 b, for example, recessed by less than 30 nm, less than 20 nm, less than 15 nm, or less than 10 nm, for example, recessed in a range of 2 nm to 20 nm, or in a range of 4 nm to 10 nm. In various embodiments, prior to direct bonding, the recesses in the opposing elements can be sized such that the total gap between opposing contact pads is less than 15 nm, or less than 10 nm. The non-conductive bonding layers 108 a and 108 b can be directly bonded to one another without an adhesive at room temperature in some embodiments and, subsequently, the bonded structure 100 can be annealed. Upon annealing, the conductive features 106 a and 106 b can expand and contact one another to form a metal-to-metal direct bond. Beneficially, the use of Direct Bond Interconnect, or DBI®, techniques commercially available from Adeia of San Jose, CA, can enable high density of conductive features 106 a and 106 b to be connected across the direct bond interface 118 (e.g., small or fine pitches for regular arrays). In various embodiments, the conductive features 106 a and 106 b and/or traces can comprise copper or copper alloys, although other metals may be suitable. For example, the conductive features disclosed herein, such as the conductive features 106 a and 106 b, can comprise fine-grain metal (e.g., a fine-grain copper). In specific embodiments described hereinbelow, at least one of the conductive features 106 a and 106 b is predominantly aluminum or includes a predominantly aluminum portion.

Thus, in direct bonding processes, a first element 102 can be directly bonded to a second element 104 without an intervening adhesive. In some arrangements, the first element 102 can comprise a singulated element, such as a singulated integrated device die. In other arrangements, the first element 102 can comprise a carrier or substrate (e.g., a wafer) that includes a plurality (e.g., tens, hundreds, or more) of device regions that, when singulated, form a plurality of integrated device dies. Similarly, the second element 104 can comprise a singulated element, such as a singulated integrated device die. In other arrangements, the second element 104 can comprise a carrier or substrate (e.g., a wafer). The embodiments disclosed herein can accordingly apply to wafer-to-wafer (W2W), die-to-die (D2D), or die-to-wafer (D2W) bonding processes. In wafer-to-wafer (W2W) processes, two or more wafers can be directly bonded to one another (e.g., direct hybrid bonded) and subsequently singulated using a suitable singulation process. After singulation, side edges of the singulated structure (e.g., the side edges of the two bonded elements) may be substantially flush and may include markings indicative of the common singulation process for the bonded structure (e.g., saw markings if a saw singulation process is used).

As explained herein, the first and second elements 102 and 104 can be directly bonded to one another without an adhesive, which is different from a deposition process and results in a structurally different interface compared to a deposition. In one application, a width of the first element 102 in the bonded structure is similar to a width of the second element 104. In some other embodiments, a width of the first element 102 in the bonded structure 100 is different from a width of the second element 104. Similarly, the width or area of the larger element in the bonded structure may be at least 10% larger than the width or area of the smaller element. The first and second elements 102 and 104 can accordingly comprise non-deposited elements. Further, directly bonded structures 100, unlike deposited layers, can include a defect region along the bond interface 118 in which nanometer-scale voids (nanovoids) are present. The nanovoids may be formed due to activation of the bonding surfaces 112 a and 112 b (e.g., exposure to a plasma). As explained above, the bond interface 118 can include concentration of materials from the activation and/or last chemical treatment processes. For example, in embodiments that utilize a nitrogen plasma for activation, a nitrogen peak can be formed at the bond interface 118. The nitrogen peak can be detectable using secondary ion mass spectroscopy (SIMS) techniques. In various embodiments, for example, a nitrogen termination treatment (e.g., exposing the bonding surface to a nitrogen-containing plasma) can replace OH groups of a hydrolyzed (OH-terminated) surface with NH₂molecules, yielding a nitrogen-terminated surface. In embodiments that utilize an oxygen plasma for activation, an oxygen peak can be formed at the bond interface 118. In illustrated embodiments described hereinbelow, employing fluorine treatment for termination, a fluorine peak can be formed at the bond interface 118. In some embodiments, the bond interface 118 can comprise silicon oxynitride, silicon oxycarbonitride, or silicon carbonitride. As explained herein, the direct bond can comprise a covalent bond, which is stronger than van Der Waals bonds. The bonding layers 108 a and 108 b can also comprise polished surfaces that are planarized to a high degree of smoothness.

In various embodiments, the metal-to-metal bonds between the conductive features 106 a and 106 b can be joined such that metal grains grow into each other across the bond interface 118. In some examples of direct hybrid bonding, the metal is or includes copper, which can have grains oriented along the 111 crystal plane for improved copper diffusion across the bond interface 118. In some examples of direct hybrid bonding, the conductive features 106 a and 106 b may include nanotwinned copper grain structure, which can aid in merging the conductive features during anneal. In illustrated embodiments described hereinbelow, one or both of the conductive features 106 a and 106 b comprise fluorine-treated aluminum. The bond interface 118 can extend substantially entirely to at least a portion of the bonded conductive features 106 a and 106 b, such that there is substantially no gap between the non-conductive bonding layers 108 a and 108 b at or near the bonded conductive features 106 a and 106 b. In some embodiments, a barrier layer may be provided under and/or laterally surrounding the conductive features 106 a and 106 b (e.g., which may include copper). In other embodiments, however, there may be no barrier layer under the conductive features 106 a and 106 b, for example, as described in U.S. Pat. No. 11,195,748, which is incorporated by reference herein in its entirety and for all purposes.

Beneficially, the use of the direct hybrid bonding techniques described herein can enable extremely fine pitch between adjacent conductive features 106 a and 106 b, and/or small pad sizes. For example, in various embodiments, the pitch p (i.e., the distance from edge-to-edge or center-to-center, as shown in FIG. 1A) between adjacent conductive features 106 a (or 106 b) can be in a range of 0.5 microns to 100 microns, 0.5 microns to 50 microns, in a range of 0.75 microns to 25 microns, in a range of 1 micron to 25 microns, in a range of 1 micron to 10 microns, or in a range of 1 micron to 5 microns. Further, a major lateral dimension (e.g., a pad diameter) can be small as well, e.g., in a range of 0.25 microns to 30 microns, in a range of 0.25 microns to 5 microns, or in a range of 0.5 microns to 5 microns.

As described above, the non-conductive bonding layers 108 a, 108 b can be directly bonded to one another without an adhesive and, subsequently, the bonded structure 100 can be annealed. Upon annealing, the conductive features 106 a, 106 b can expand and contact one another to form a metal-to-metal direct bond. In some embodiments, the materials of the conductive features 106 a, 106 b can interdiffuse during the annealing process.

FIG. 2 is a schematic cross-sectional side view of a bonded structure 1. The bonded structure 1 includes a first element 10 and a second element 12 bonded by way of solder balls 14. The first element 10 includes a first back end of line (BEOL) layer 16 and aluminum pads 18. A polymer layer 20 can be disposed over a surface of the BEOL layer 16 for supporting the solder balls 14. The second element 12 includes a second BEOL layer 26 and aluminum pads 28. The solder balls 14 are provided with the first element 10 and bonded to the aluminum pads 28 of the second element 12.

Using the solder balls 14 may not be feasible for relatively fine pitch interconnects, such as an interconnect pitch of less than 20 microns, or less than 2 microns. Additionally, because the mechanical connection between the elements 10 and 12 is limited to the solder joints, the mechanical connection can be weak and subject to separation from physical stresses or shocks because of the brittle intermetallic compound (IMC) typically formed with the solder balls. After bonding, the gap between the top surfaces of the BEOL layers 16, 26 are cleaned and filled with a dielectric underfill material (not shown) to encapsulate the solder balls 14 and corresponding contacts. The resistivity of the alloy formed with the solder which is typically at least 3 times higher than that of an aluminum pad or copper post, can cause electrical losses. Also, the dielectric underfill that mechanically connects the top surfaces of the BEOL layers 16, 26 impedes heat transfer between the first element 10 and the second element 12. Therefore, it can be beneficial to provide a bond surface that can directly hybrid bond the elements 10, 12 together.

FIGS. 3A-3E show a method of forming copper pads 32 over aluminum pads or interconnects 34 formed with a BEOL layer 16 of an element 30. At FIG. 3A, the BEOL layer 16 can be provided. The BEOL layer 16 can include a planar dielectric surface and aluminum pads or interconnects 34 that are at least partially embedded in the dielectric. As shown in FIG. 3B, the method includes forming a planarized dielectric layer 36 over the BEOL layer 16. At FIG. 3C, cavities 38 are formed in the dielectric layer 36. The aluminum pads or interconnects 34 can be exposed through the cavities 38.

At FIG. 3D, a barrier layer 40 and/or a seed layer can be formed over the dielectric layer 36 and surfaces of the cavities 38. Copper 42 is provided at least in the cavities over the barrier layer 40. At FIG. 3D copper 42 is overfilled and excess copper 42 is present over the surface of the dielectric layer 36. The element 30 may be annealed at a temperature, for example, below 200° C., to at least partially stabilize the microstructure of copper 42, if needed. The excess copper 42 can be removed in FIG. 3E to form the copper pads 32. The excess copper 42 can be removed, for example, by way of polishing, such as chemical mechanical planarization (CMP). The polishing process can also remove portions of the barrier layer 40 from over the surface of the dielectric layer 36, and form a direct bonding surface of the element 30 after activation and/or termination as described above.

As shown in FIGS. 4A-4E, aluminum pads 44 can be formed in a similar manner as the method of forming the copper pads 32 shown in FIGS. 3A-3E. In place of the copper 42 provided in FIG. 3D, aluminum 46 can be provided in FIG. 4D, and excess aluminum 46 can be removed to form the aluminum pads 44. Alternatively, a blanket layer of aluminum can be deposited over the structure of FIG. 4A and patterned with an appropriate masking process and the unwanted portions of the aluminum subsequently etched (e.g., by reactive ion etch). For the alternative process, the dielectric bonding layer 36 is deposited over the patterned aluminum layer and polished back to the aluminum pads 44 to form a smooth dielectric bonding surface.

One disadvantage of the processes of FIGS. 3A-4E is the separate deposition of a bonding layer 36, which often entails exposing the elements to high temperature processing, and an expensive masking process to define the cavities 38. Moreover, for the process of FIGS. 4A-4E, the aluminum pads 44 are susceptible to oxidation, and a surface oxide may be formed over the aluminum pads 44. Also, the aluminum pads 44 are exposed to higher temperatures typically greater than 250° C. and usually 300° C. to 350° C. when the bonding layer 36 is deposited over the pre-patterned aluminum pads 44. The higher temperature dielectric process induces large grain formation in the aluminum pads 44. The subsequent dielectric planarization step forms a smooth dielectric bonding surface comprising surfaces of the aluminum pads 44 with the large grains. Metal pads with large grains (0.5 to 3 microns or larger) have fewer grain boundaries compared to metal pads with finer grains, for example less than 0.3 microns. In practice, pads with finer metal tend to bond at lower temperatures than pads with large grains. The susceptibility of aluminum to form thin surface aluminum oxide during planarization process and upon exposure to ambient air can be problematic during the direct bonding operation step. The surface oxide on the aluminum pads 44 impedes direct bonding of the aluminum pads 44 or 34 with corresponding aluminum pads of another element. Therefore, some treatment, such as argon sputtering may be employed to remove the surface oxide prior to bonding. However, such process may be time consuming and costly. Such process may also redeposit aluminum particles over portions of the dielectric bonding surface. One solution to avoid surface oxidation is to activate the surfaces of the aluminum pads 44 or 34 with nitrogen plasma. The exposure of the aluminum pads 44, 34 to nitrogen plasma causes aluminum nitride to be formed on the surfaces of the aluminum pads 44. While aluminum nitride at the surface is easier to deal with for direct bonding than aluminum oxide, such processes may call for a high temperature (e.g., 300° C. to 400° C.) bonding process in order to allow the surface aluminum nitride to decompose or for aluminum to diffuse through the surface nitride and form a metallurgical joint between pads 34, 44 of the opposing elements. The high temperature bonding process increases the thermal budget for forming a bonded structure. Various embodiments disclosed herein can achieve more simplified, cost-effective methods and structures for forming a bonded structure.

FIGS. 5A to 5F show a method of forming a bonding surface 76 a of an element 76 according to an embodiment. FIG. 5A is a schematic cross-sectional side view showing an aluminum layer 60 over a back end of line (BEOL) layer 62, which is formed over devices (not shown) of the element, such as in or on a semiconductor material. A barrier layer 68 such as TiN, TiN/Ti, TiW or TiW/Ti, and/or a seed layer can be disposed between the BEOL layer 62 and the aluminum layer 60. The BEOL layer 62 can include a dielectric region 64 and an interconnect structure 66. The interconnect structure 66 can comprise an aluminum interconnect. The barrier layer 68 can be deposited over a surface of the BEOL layer 62, and the aluminum layer 60 can be deposited over the barrier layer 68. For example, the aluminum layer 60 can be provided by way of sputtering. For example, the aluminum layer 60 can be sputter deposited at about 150° C. or lower. The aluminum layer 60 can be relatively thin. For example, a thickness of the aluminum layer 60 can be in a range of 0.5 μm to 7 μm, 0.5 μm to 5 μm, 0.5 μm to 4 μm, 1 μm to 5 μm, 1 μm to 3 μm, or 1 μm to 2 μm depending at least in part on the circuit requirements.

In FIG. 5B, a selected or desired masking layer, such as a resist layer 70, can be provided over the aluminum layer 60 by lithographic methods. The resist layer 70 can be patterned over the aluminum layer 60 such that portions of the resist layer 70 are positioned over the interconnect structure 66.

In FIG. 5C, portions of the aluminum layer 60 can be selectively removed and form conductive features 72 by, for example, reactive ion etching (RIE) methods or by wet etch. For example, the portions of the aluminum layer 60 that are not covered by or free from the resist layer 70 can be etched using any suitable etching process, such as plasma etching, including reactive ion etching, to define the conductive features 72. The conductive feature 72 is an example of a conductive feature. The conductive features 72 can be conductive metal pads, vias or lines that include aluminum more than 50% by volume. For example, the conductive features 72 can comprise more than 80%, more than 90%, or more than 95% of aluminum by volume. In the illustrated embodiment, the conductive features 72 are patterned from the aluminum layer 60 and thus can be considered aluminum features, but in other embodiments the conductive features 72 may comprise one or more other metal layers, while still constituting singular features defined by a single mask and still comprising more than 50% aluminum by volume. Thus, each of the conductive features 72 may have continuous sidewalls characteristic of definition by a single mask process.

In FIG. 5D, a dielectric layer 74 can be provided over the conductive features 72 and over portions of the BEOL layer 62. In some embodiments, the dielectric layer 74 can comprise an oxide layer. For example, the oxide layer can be deposited at about 350° C. or other desirable temperatures by known methods. In some embodiments, multiple dielectric coating steps with other intermediary process(es) may be applied to form the dielectric layer 74. The dielectric coating may comprise a conformal coating (as shown) or nonconformal coating. In some embodiments, the dielectric layer 74 may comprise a combination of conformal and nonconformal dielectric coatings. The dielectric layer 74 can be referred to as a nonconductive field region. In some embodiments, the dielectric layer 74 is formed by sputtering, spin-on-deposition or other low temperature process.

As shown in FIG. 5E, portions of the dielectric layer 74 can be removed (e.g., polished) to expose surfaces of the conductive features 72. The surfaces of the conductive features 72 and surface of the remaining dielectric layer 74 can define the bonding surface 76 a of the element 76. The bonding surface 76 a of the element 76 can be polished using, for example, chemical mechanical polishing (CMP), as disclosed herein. Either the CMP chemistry or a subsequent selective etch can recess the surface of the conductive features 72 below the dielectric layer 74. The roughness of the polished bonding surface 76 a can be less than 30 Å rms. For example, the roughness of the bonding surface 76 a can be in a range of about 0.1 Å rms to 15 Å rms, 0.5 Å rms to 10 Å rms, or 1 Å rms to 5 Å rms. Additionally, the conductive features 72 and surrounding dielectric layer 74 can be formed during the back-end-of-line process, such that element 76 may be as supplied by an integrated circuit manufacturer, before or after singulation. A thickness of the conductive features 72 can be in a range of about 0.5 μm to 7 μm or 1 μm to 5 μm.

In FIG. 5F, the bonding surface 76 a of the element 76 can be terminated. Although not shown, it will be understood that a protective layer can be provided and removed between polishing of FIG. 5E and the termination of FIG. 5F, similarly to that described below with respect to FIG. 6I. The bonding surface 76 a of the element 76 can be terminated with, for example, fluorine to define a very thin layer of continuous or discontinuous surface aluminum fluoride (Al—F) complex, aluminum fluoride oxide (Al—F—O) complex, or aluminum fluoride boron oxide (Al—F—B—O) complex. In some embodiments, the bonding surface 76 a can be rinsed by a rinsing solution that supplies the surface termination. For example, the rinsing solution can comprise 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium tetrafluoroborate, tetramethylammonium tetrafluoroborate, tetramethylammonium tetrafluoroborate, hydrogen fluoride (HF), or any suitable combination thereof. After termination, the bonding surface 76 a can be spin dried, with or without a prior deionized water rinse. In some embodiments, the fluorine termination may be accomplished by way of a plasma process. For example, the surface of the conductive feature can be exposed to a gentle fluoride plasma or placed in a chamber where fluorine bearing gas such as carbon tetrafluoride (CF₄) has been used. The residual fluorine on the walls of the chamber can be adsorbed on the bonding surface 76 a. The fluoride may reduce some of the native aluminum oxide. In some embodiments, the bonding surface 76 a of the element 76 is not exposed to nitrogen plasma and/or ammonium dip before and after the fluorine treatment.

After the bonding surface 76 a of the element 76 has been terminated, there can be gradient of fluorine concentration from the bonding surface 76 a into the element 76. Thus, the element 76 can have a higher fluorine content at the bonding surface 76 a than portions of the element 76 farther away from the surface. For example, there may be one or multiple fluorine peaks at or near the bonding surface 76 a. In some embodiments, the conductive features 72 can have a gradient of fluorine concentration such that the surface of the conductive feature 72 has a higher fluorine content compared to deeper into the conductive feature. For example, the conductive feature 72 can have the highest fluorine concentration at the surface and the fluorine concentration gradually decreases from the surface of the conductive feature 72 to a portion of the conductive feature 72 farther away from the surface. In some embodiments, fluorine can be present about 4 nm to 6 nm (e.g., 5 nm) below the surface. In some embodiments, the level of oxygen present on the surface of the conductive feature 72 can very low or undetectable. For example, there may be less than 20 ppm of oxygen present on the surface of the conductive feature 72. In some embodiments, the bonding surface 76 a can include a portion that is free from aluminum oxide. In some embodiments, very low or no carbon and/or nitrogen can be present in the conductive feature 72 and/or the dielectric layer 74. For example, there may be less than 50 ppm, 40 ppm, or 20 ppm of carbon or nitrogen present on the surface of the conductive feature 72 or the dielectric layer 74.

Without such termination, aluminum oxide (Al₂O₃) can be formed on the surfaces the conductive features 72 even upon room temperature exposure to air, including clean room air. Terminating the bonding surface 76 a with fluoride can suppress or prevent aluminum oxide (Al₂O₃) formation on the surface of the conductive features 72. Table 1 below shows melting points and thermal expansion rates of aluminum (Al), aluminum oxide (Al₂O₃), aluminum nitride (AlN), and aluminum fluoride (AlF₃).

TABLE 1

Materials	Melting point	Thermal Expansion

Al	660°	C.	23	ppmK⁻¹
Al₂O₃	2977°	C.	4.5	ppmK⁻¹
AlN	2200°	C.	5.3	ppmK⁻¹

	AlF₃	1250°	C.	α_v- 86 ppmK⁻¹

Though aluminum has the lowest melting point among the four materials in Table 1 and the thermal expansion rate is relatively high, aluminum is susceptible to forming aluminum oxide at its surface with any air exposure. Aluminum oxide has a significantly higher melting point than the other three materials (about 4.5 times the melting point of aluminum) and has a significantly lower thermal expansion rate. Aluminum nitride has a lower melting point and a slightly higher thermal expansion rate than aluminum oxide. Nevertheless, the melting point of aluminum nitride is still significantly higher than aluminum and the thermal expansion rate of aluminum nitride is still significantly lower than aluminum. Aluminum fluoride has a comparatively lower melting point and a significantly high thermal expansion rate. Accordingly, terminating the surfaces of the conductive features 72 with fluorine can provide a reliable bonding surface (the bonding surface 76 a) of the element 76 and facilitate lower anneal temperatures for forming the metal bonds during direct hybrid bonding. In some conditions, the fluorine terminated surface can mitigate or prevent oxide formation on the surface of the conductive features 72 for up to, for example, three days, four days, a week, or ten days.

The element 76 can be a wafer or a die, and the element 76 can be bonded to a second element (e.g., a wafer or a die). FIG. 5G is a schematic cross sectional side view of a bonded structure 2 that includes the element 76 in form of a wafer and a second element 80 (another wafer) bonded to the element 76 along a bond interface 82. In some embodiments, the second element 80 may be bonded to another substrate of interest. FIG. 5H is a schematic cross sectional side view of a bonded structure 2′ that includes the element 76 in form of a wafer and a plurality of second elements 84 (dies) bonded to the element 76 along bond interfaces 86. The second elements 80 or 84 can include conductive features 92 and nonconductive field regions 94, and can have a similar structure to the first element 76 (including aluminum-based conductive features 92).

The bonding surface 76 a of the element 76 can be directly bonded to bonding surface(s) 80 a, 84 a of the second element 80, 84 in any suitable manner disclosed herein. The bonding surface 76 a of the element 76 can be directly bonded to bonding surface(s) 80 a, 84 a the second element(s) 80, 84 such that the nonconductive field region(s) 94 of the second element(s) 80, 84 are directly bonded to the dielectric layer 74 of the element 76 (which can be considered a first element in the embodiments of FIGS. 5H and 5G). This initial direct bond of the dielectric materials can form strong covalent bonds between the nonconductive field regions 94 of the opposing elements 76, 80, 84 at room temperature. In some embodiments, the second element 84 may be bonded to another substrate of interest.

The bonded elements 76 and 80, or 76 and 84 can then be heated (e.g., annealed) in order to cause the conductive features 72, 92 to expand across the gap left by the recessed aluminum, contact one another and form direct metal bonds. In some embodiments, the bonded elements 76 and 80, or 76 and 84 can be annealed at a temperature lower than 350° C., for example a temperature lower than or equal to about 300° C. or a temperature lower than or equal to about 250° C., for less than or equal to about 4 hours, such as between about 2.5 hours and 4 hours. For example, the annealing temperature can be in a range between about 150° C. and 300° C., between about 200° ° C. and 300° C., or between about 200° ° C. and 250° C. The annealing process can strengthen the bond between the conductive features 72 and the conductive features 92.

A fluorine content can be higher at the bonding surface 76 a compared to deeper into the elements 76 and 80, 76 and 84. A gradient of fluorine concentration at or near the bonding surface 76 a can be present in the bonded structure 2, 2′. For example, in the bonded structure 2, 2′, there may be one or multiple fluorine peaks at or near a bond interface 82, 86 between the first and second elements 76 and 80, 76 and 84. In some embodiments, there is a very low or indetectable level of oxygen present between the conductive feature 72 and the conductive feature 92 after metal bonding. For example, there may be less than 1000 ppm, less than 500 ppm, or less than 100 ppm of oxygen present between the conductive feature 72 and the conductive feature 92 after bonding. Similarly, very low or no carbon and/or nitrogen associated with the terminating molecule or molecules can be present between the conductive feature 72 and the conductive feature 92 and/or between the dielectric layer 74 the nonconductive field region 94 after bonding. For example, there may be less than 100 ppm or less than 80 ppm of nitrogen and/or carbon present between the conductive feature 72 and the conductive feature 92 after bonding.

FIGS. 6A to 6E show a method of forming a bonding surface 150 a of an element 150 according to an embodiment. FIG. 6A is a schematic cross-sectional side view showing an aluminum layer 73 comprising large aluminum metal grains or fine aluminum grains and a dielectric layer 74 formed over a back end of line (BEOL) layer 62 that includes an interconnect structure 66. The aluminum layer 73 and/or the dielectric layer 74 can be planar. The structure of FIG. 6A can be the same as or generally similar to the structure of FIG. 5E. As described above, the aluminum layer 73 and the dielectric layer 74 can also be part of the BEOL or a redistribution layer (RDL) structure of an integrated device.

In FIG. 6B, at least a portion of the aluminum layer 73 can be selectively removed to form a recess 152 over the aluminum layer 73, such as by RIE, wet etch, or other known methods. A depth of the recess can be in a range of 0.1 μm to 0.5 μm, 0.1 μm to 0.25 μm, or 0.15 μm to 0.2 μm. The portion of the aluminum layer 73 can be removed by way of, for example, dry etching (e.g., vapor or plasma etching) or wet etching.

In FIG. 6C, a barrier layer 153 and a fine grain aluminum layer 154 can be provided in the recess 152 and over the surface 74 a of the dielectric layer 74. For example, the aluminum with fine grain microstructure may be formed over the recess 152 by sputtering and cooling the BEOL layer 62, or cooling a substrate (not shown) to which the BEOL layer 62 is bonded, below 100° C. and preferably below 50° C. or below, such as 20° ° C., during the sputtering. In some embodiments, the barrier layer 153 can completely or partially separate the fine grain aluminum layer 154 from the aluminum layer 73 and the dielectric layer 74. In some embodiments, the barrier layer 153 may be omitted, and the fine grain aluminum layer 154 can be coated directly over the larger grain aluminum 73. Fine grain aluminum can be defined as aluminum having an average grain dimension (e.g., width) less than 15 nm, less than 20 nm, less than 50 nm, less than 100 nm, less than 200 nm, less than 300 nm, or less than 500 nm. For example, the maximum width of grains in the fine grain aluminum layer 154 can be in a range of about 10 nm to 500 nm, about 10 nm to 300 nm, about 15 nm to 500 nm, about 15 nm to 300 nm, about 15 nm to 100 nm, about 15 nm to 50 nm, about 50 nm to 500 nm, about 50 nm to 300 nm, or about 100 nm to 300 nm. In some embodiments, most of the grains in the fine grain aluminum layer 154 can have a width in a range of about 10 nm to 500 nm, about 10 nm to 300 nm, about 15 nm to 500 nm, about 15 nm to 300 nm, about 15 nm to 100 nm, about 15 nm to 50 nm, about 50 nm to 500 nm, about 50 nm to 300 nm, or about 100 nm to 300 nm.

In some embodiments, the fine grain aluminum layer 154 can be provided by way of a low temperature deposition. For example, the fine grain aluminum layer 154 can be deposited at a temperature less than about 100° C., less than about 65° C., or less than about 20° C. For example, a deposition temperature for depositing the fine grain aluminum layer 154 can be in a range between about 10° C. and 100° C., about 10° C. and 65° ° C., about 10° C. and 50° C., or about 10° C. and 20° C. A thickness of the fine grain aluminum layer 154 can be the same as or thicker than the depth of the recess 152.

In FIG. 6D, portions of the barrier layer 153 and the fine grain aluminum layer 154 over the surface 74 a of the dielectric layer 74 can be removed. For example, the portion of the barrier layer 153 and the fine grain aluminum layer 154 can be removed by way of polishing (e.g., chemical mechanical polishing (CMP)) to a degree sufficient to define the bonding surface 150 a of the element 150 that includes the surface 74 a of the dielectric layer 74 and a surface 154 a of the fine grain aluminum layer 154. Either the CMP chemistry or a subsequent etch process can recess the aluminum surface 154 a below the dielectric surface 74 a. The CMP process can take place at a temperature lower than the deposition temperature for depositing the fine grain aluminum layer 154. The aluminum layer 73, the barrier layer 153, and the fine grain aluminum layer 154 can together define a conductive feature 155. As with the prior embodiment, the conductive features 155 may comprise more than 50% aluminum by volume, for example greater than 80% aluminum by volume, may have continuous sidewalls characteristic of definition by a single mask process. Where predominantly aluminum, as in the illustrated embodiment, the conductive feature 155 can be referred to as an aluminum feature, in some embodiments. The conductive feature 155 can have a first portion (e.g., the aluminum layer 73) and a second portion (e.g., the fine grain aluminum layer 154). The second portion can have a microstructure different from the first portion. A thickness of the conductive feature 155 can be in a range of about 0.5 μm to 7 μm or 1 μm to 5 μm.

In FIG. 6E, the bonding surface 150 a of the element 150 can be terminated in the same or similar manner disclosed above with respect to FIG. 5F. The bonding surface 150 a of the element 150 can be terminated with, for example, fluorine to define surface aluminum fluoride complex, aluminum fluoride oxide complex, or aluminum fluoride boron oxide complex. In some embodiments, the bonding surface 150 a can be rinsed by a rinsing solution. For example, the rinsing solution can comprise 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium tetrafluoroborate, tetramethylammonium tetrafluoroborate, tetramethylammonium tetrafluoroborate, hydrogen fluoride (HF), buffered hydrogen fluoride (BHF), or any suitable combination thereof. The bonding surface 150 a can be spin dried without rinsing with deionized (DI) water. In some embodiments, the treated surface may be rinsed with DI water or other suitable solvents and dried by known methods, for example, by spin drying. In some embodiments, the fluorine termination may be accomplished by way of a plasma process as described above.

The bonding surface 150 a of the element 150 can be formed without a high temperature process. Therefore, the relatively small grain sizes of the fine grain aluminum layer 154 can be maintained, which can be advantageous for subsequent metal bonding.

FIG. 6F is a schematic cross-sectional side view of a bonded structure 3 that includes the element 150 and a second element 160 bonded to the element 150. In some embodiments, the second element 160 can have the same or generally similar structure as the element 150. The second element 160 can include a BEOL layer 162 including interconnect features, a dielectric layer 164, and conductive features 165 that includes an aluminum layer 166 and a fine grain aluminum layer 168 that together can serve as a pad. The bonding surface 150 a of the element 150 can be directly bonded to a bonding surface 160 a of the second element 160 along a bond interface 170 such that the dielectric layer 74 is directly bonded to the dielectric layer 164. This initial direct bond of the dielectric materials can form strong covalent bonds between the dielectric layers 74, 164 of the opposing elements 150, 160 at room temperature. In some embodiments, the second element 160 may be bonded to another substrate of interest.

In FIG. 6G, the bonded structure 3 formed in FIG. 6F can be heated (e.g., annealed). In some embodiments, the structure formed in FIG. 6F can be annealed at a temperature lower than about 350° C., lower than about 300° C. or a temperature lower than about 250° C. for less than or equal to about 2 hours. For example, the annealing temperature can be in a range between about 150° C. and 300° C., between about 200° C. and 300° C., or between about 200° C. and 250° C. The annealing process can cause expansion of the fine grain aluminum layers 154, 168 across the gap left by the recessed aluminum surfaces and cause metal bonding between the fine grain aluminum layers 154, 168. Annealing can also strengthen the bond between the dielectric layers 74, 164. The grains of the fine grain aluminum layer 154 can grow due to the heating process. In some embodiments, the grains of the fine grain layer 154 can still be smaller than the grains of the aluminum layer 73. For example, an average grain dimension (e.g., width) of the grains in the fine grain aluminum layer 154 after annealing can be less than 1000 nm, 750 nm or 500 nm. For example, the maximum width of the largest grain in the fine grain aluminum layer 154 after annealing can be in a range of about 20 nm to 500 nm. In some embodiments, most of the grains in the fine grain aluminum layer 154 after annealing can have a width in a range of about 200 nm to 500 nm. In some embodiments, the fine grain aluminum layer 154 can be deposited only on one of the bonded substrates. In some embodiments, the fine grain aluminum layer 154 may comprise aluminum nanoparticles formed by physical vapor deposition (PVD) methods or atomic layer deposition (ALD) or other known methods.

Like the element 76, the element 150 can be a die or a wafer and the second element 160 can be a die or a wafer. Thus, the process for bonding the element 150 and the second element 160 can include wafer-to-wafer (W2W), die-to-die (D2D), or die-to-wafer (D2W) bonding processes. In some embodiments, the bonded structure 3 can include additional wafers, substrates, or dies stacked and bonded over the second element 160. The stacked elements can be electrically connected via, for example, various TSVs.

FIG. 6H is a cross-sectional side view of singulated bonded structures 3 a, 3 b, 3 c. In some embodiments, the singulated bonded structures 3 a, 3 b, 3 c can be formed by W2W bonding the element 150 and the second element 160 of FIG. 6F followed by a singulation process. For example, the singulation process can include attaching a bonded W2W structure to a dicing film or tape 172, forming a protective layer 174 on the bonded W2W structure, and singulating the bonded W2W structure into a plurality of singulated structures, such as the singulated bonded structures 3 a, 3 b, 3 c. Each of the singulated bonded structures 3 a, 3 b, 3 c can include a die from the element 150 and a die from the second element 160.

FIG. 6I is a cross-sectional side view of singulated elements 151 a, 151 b, 151 c after at least partial preparation for direct hybrid bonding but before actual bonding. In some embodiments, the element 150 shown in FIG. 6D can comprise a wafer from which the singulated elements 151 a, 151 b, 151 c can be formed. For example, the element 150 in form of a wafer can be positioned on a dicing film or tape 172. A protective layer 174 can be formed on the element 150. The element 150 can be singulated into the singulated elements 151 a, 151 b, 151 c. After singulation, the protective layer 174 can be removed as shown in FIG. 6J.

After removing the protective layer 174, as shown in FIG. 6K, bonding surfaces of the singulated elements 151 a, 151 b, 151 c can be terminated as described with respect to FIG. 6F. After terminating the surfaces of the singulated elements 151 a, 151 b, 151 c, the singulated elements 151 a, 151 b, 151 c can be bonded to the second element 160 to thereby forming a D2W bonded structure 4, as shown in FIG. 6L. In the D2W bonded structure 4, the singulated elements 151 a, 151 b, 151 c can be bonded to the second element 160 as described with respect to FIG. 6G.

FIGS. 7A to 7E show a method of forming a bonding surface 200 a of an element 200 according to an embodiment. FIG. 7A is a schematic cross-sectional side view showing aluminum layer 202 and dielectric layer 74 formed over a back end of line (BEOL) or RDL layer 62 that includes an interconnect structure 66. The structure of FIG. 7A can be the same as or generally similar to the structure of FIGS. 5E and 6A.

In FIG. 7B, at least a portion of the aluminum layer 202 can be removed to form a recess 152 over each aluminum layer 202. A depth of the recess can be in a range of 0.1 μm to 0.5 μm, 0.1 μm to 0.35 μm, or 0.15 μm to 0.25 μm. The portion of the aluminum layer 202 can be removed by way of, for example, dry etching (e.g., vapor or plasma etching) or wet etching.

In FIG. 7C, a barrier layer 153 and a different type of conductive material, such as the illustrated copper layer 204 can be provided in the recess 152 and over the surface 74 a of the dielectric layer 74. The copper layer 204 is an example of the different type of conductive material and another example can include a silver layer. The copper layer 204 can comprise fine grain copper having an average grain width less than about 15 nm, less than about 20 nm, less than about 50 nm, less than about 100 nm, less than about 200 nm, less than about 300 nm, or less than about 500 nm. For example, the maximum width of grains in the copper layer 204 can be in a range of about 10 nm to 500 nm, about 10 nm to 300 nm, about 15 nm to 500 nm, about 15 nm to 300 nm, about 15 nm to 100 nm, about 15 nm to 50 nm, about 50 nm to 500 nm, about 50 nm to 300 nm, or about 100 nm to 300 nm. In some embodiments, most of the grains in the fine grain copper layer 204 can have a width in a range of about 10 nm to 500 nm, about 10 nm to 300 nm, about 15 nm to 500 nm, about 15 nm to 300 nm, about 15 nm to 100 nm, about 15 nm to 50 nm, about 50 nm to 500 nm, about 50 nm to 300 nm, or about 100 nm to 300 nm. In some embodiments, the different conductive layer may be coated by electroless or electroplating methods. Also, the different conductive layer may be coated by printing methods, or by physical vapor deposition (PVD) methods, such as evaporation or sputtering. In some embodiments, the different conductive layer may comprise nanoparticle copper, nanoparticle silver, or other nanoparticle metals. The nanoparticle metals may be coated by electroless or electrolytic or by low temperature sputtering, amongst other methods.

In some embodiments, the copper layer 204 can be provided by way of a low temperature deposition. For example, the copper layer 204 can be deposited at a temperature less than 100° C., less than 65° ° C., or less than 20° C. For example, a deposition temperature for depositing the copper layer 204 can be in a range between 1° C. and 10° C., 10° C. and 100° C., 10° C. and 65° C., 10° C. and 50° C., or 10° C. and 35° C. A thickness of the copper layer 204 can be the same as or generally similar to the depth of the recess 152. The copper layer 204 can be deposited by physical deposition (e.g., sputtered) rather than plated. By the nature of the deposition process, the copper layer 204 can have a mostly 111 texture, and a randomly oriented grain structure.

In FIG. 7D, portions of the barrier layer 153 and the copper layer 204 over the surface 74 a of the dielectric layer 74 can be removed. For example, the portion of the barrier layer 153 and the copper layer 204 over the dielectric layer 74 can be removed by way of polishing (e.g., chemical mechanical polishing (CMP)) to define the bonding surface 200 a of the element 200 that includes the surface 74 a of the dielectric layer 74 and a surface 204 a of the copper layer 204. In practice, the CMP process to form the dielectric bonding surface 200 a may polish off very small portion of the dielectric material 74 in the barrier layer 153 removal step. The CMP process can take place at a temperature lower than the deposition temperature for depositing the copper layer 204. Either the CMP chemistry or a subsequent etch process can recess the copper surface 204 a below the dielectric surface 74 a. The aluminum layer 202 and the copper layer 204 can together form a conductive feature 206. Because the upper copper layer 204 and lower aluminum layer 202 have their sidewalls or lateral extents defined by the same mask (e.g., mask for etching either aluminum to form the aluminum layer 202 or for etching dielectric layer 74 for filling with aluminum), the sidewalls of the upper and lower portions are continuous, without discontinuities (e.g., corners) typical of misalignment when the upper and lower portions are defined by separate masks. At least the lower aluminum layer 202 comprises aluminum. In the illustrated embodiment, the copper layer 204 comprises copper or other conductive material (e.g., silver), though in other embodiments the upper portion can also comprise fine grain aluminum. The conductive feature 206 can have a first portion (e.g., the aluminum layer 73) and a second portion (e.g., the upper copper layer 204). The second portion can have a microstructure different from the first portion. A thickness of the conductive feature 206 can be in a range of about 0.5 μm to 7 μm or 1 μm to 5 μm.

In FIG. 7E, the bonding surface 200 a of the element 200 can be cleaned and exposed to a plasma and/or etchants to activate the bonding surface 200 a. In some embodiments, the bonding surface 200 a of the element 200 can be terminated with a species after activation or during activation (e.g., during the plasma and/or etch processes). Without being limited by theory, in some embodiments, the activation process can be performed to break chemical bonds at the bonding surface 200 a of the element 200, and the termination process can provide additional chemical species at the bonding surface 200 a that improves the bonding energy during direct bonding. In some embodiments, the activation and termination are provided in the same step, e.g., a plasma to activate and terminate the bonding surface 200 a of the element 200. In other embodiments, the bonding surface 200 a of the element 200 can be terminated in a separate treatment to provide the additional species for direct bonding.

As explained above, the bonding surface 200 a can include concentration of materials from the activation and/or last chemical treatment processes. For example, in embodiments that utilize a nitrogen plasma for activation, a nitrogen peak can be formed at the bonding surface 200 a. The nitrogen peak can be detectable using secondary ion mass spectroscopy (SIMS) techniques.

In various embodiments, for example, a nitrogen termination treatment (e.g., exposing the bonding surface to a nitrogen-containing plasma) can replace OH groups of a hydrolyzed (OH-terminated) surface with NH₂molecules, yielding a nitrogen-terminated surface. In embodiments that utilize an oxygen plasma for activation, an oxygen peak can be formed at the bonding surface 200 a. In some embodiments, the bonding surface 200 a can comprise silicon oxynitride, silicon oxycarbonitride, or silicon carbonitride.

As with the elements 76, 150 of prior embodiments, the element 200 can be a die or a wafer, and the element 200 can be bonded to another element (e.g., a wafer or a die). FIG. 7F is a schematic cross sectional side view of a bonded structure 5 that includes the element 200 in form of a wafer and a second element 220 (another wafer) bonded to the element 200 along a bond interface 221. The second element 220 can comprise a BEOL layer 222, a dielectric layer 224, and a conductive feature 232 that can also include a lower portion 228 (e.g., aluminum) and an upper portion 230 (e.g., copper) as shown. The dielectric layer 74 and the dielectric layer 224 can be directly bonded to one another without an intervening adhesive, and the copper layer 204 and the upper layer 230 (e.g., copper layer) can be directly bonded to one another without an intervening adhesive. In other embodiments, additional wafers or substrates (e.g., 1 to 8 wafers) may be mechanically and electrically bonded over the second element 220. The bonded wafer or substrate stack may be singulated for subsequent processes. Similarly, additional dies (e.g., 1 to 40 dies) may be bonded or stacked over the second element 220. The wafer or substrate with bonded dies can be singulated for subsequent processing. One of the subsequent processes can comprise encapsulating the sides of the singulated stack in a dielectric layer. In some embodiments, the encapsulating dielectric layer comprises a molding material.

As with the element 150 illustrated in FIGS. 61 and 6J, the element 200 can be singulated into singulated elements prior to bonding, in some embodiments. The singulated elements can be bonded to another element (e.g., a wafer or a die) as described herein.

Various embodiments disclosed herein, or features thereof, can be combined to provide further embodiments. For example, any two or more elements disclosed herein can be bonded to define various bonded structures. For example, any two or more of the elements 76, 150, 200 can be bonded to define a bonded structure.

In the foregoing embodiments, aluminum-based features, such as contact pads, can be treated for efficient direct metal bonding, including direct hybrid bonding. In the illustrated embodiments, an aluminum feature can be defined and, without additional high temperature depositions or masking steps, treated for direct hybrid bonding. For example, unlike the process of FIGS. 3A-3D, no masking steps or insulating layer depositions are shown after the aluminum features are defined (e.g., at the stage of FIG. 5E). Consequently, in all of the illustrated embodiments, a metal contact feature prepared for direct hybrid bonding can have a continuous sidewall, characteristic of definition with a single masking process, with a lower aluminum portion and either an upper aluminum portion with fluorine treatment or an upper copper portion. In the embodiments of FIGS. 5A-5H, no depositions or masking steps are shown after definition of the aluminum feature at the surface, and fluorination facilitates low temperature direct hybrid bonding of the aluminum feature. The embodiments of FIGS. 6A-6L and 7A-7F can employ recessing and redeposition of metal to form different upper portions of the contact feature compared to lower portions (e.g., different aluminum grain structures or copper on top and aluminum on the bottom), but even these embodiments do not rely on additional insulator depositions or mask steps, such that the contact feature having a lower aluminum portion and a different metal upper portion still has a continuous sidewall characteristic of definition by a single masking process. Furthermore, the recessing, metal redeposition and CMP steps of FIGS. 6A-7F can all be performed at low temperatures compared to oxide deposition.

In one aspect, a method of forming a bonding surface for direct hybrid bonding is disclosed. The method can include providing an element having a nonconductive field region and an aluminum feature, and exposing a surface of the aluminum feature to fluorine. The surface of the aluminum feature and the surface of the nonconductive field region define the direct bonding surface.

In one embodiment, exposing the surface of the aluminum feature to fluorine suppresses aluminum oxide formation.

In one embodiment, the method further includes providing an aluminum layer over a back-end-of-line (BEOL) layer, removing at least a portion of the aluminum layer to define the aluminum feature, and providing a dielectric material proximate to the aluminum feature to define the nonconductive field region.

In one embodiment, the aluminum feature includes a first portion and a second portion over the first portion and at least partially defining the surface of the aluminum feature. The second portion can include an average grain size smaller than an average grain size of the first portion. The method can further include removing metal from an initial aluminum feature to leave the first portion below a recess of about 0.1 μm to 0.3 μm relative to the surface of the nonconductive field region, and depositing the second portion into the recess over the first portion. The second portion can have a microstructure different from the first portion. The second portion can be deposited at a deposition temperature below about 100° C. Most of the grains in the second portion can have a width in a range of 10 nm to 500 nm. The aluminum feature can have a continuous sidewall along a sidewall of the first portion and a sidewall of the second portion.

In one embodiment, a thickness of the aluminum feature is in a range of about 0.5 μm to 7 μm. The thickness of the aluminum feature can be in a range of about 1 μm to 5 μm.

In one embodiment, the element includes an aluminum interconnect structure electrically connected to the aluminum feature.

In one embodiment, exposing the surface of the aluminum feature to fluorine includes forming aluminum fluoride, aluminum fluoride oxide, or aluminum fluoride boron oxide. Exposing the surface of the aluminum feature to fluorine can include forming at least a portion that is free from aluminum oxide.

In one embodiment, exposing the surface of the aluminum feature to fluorine includes exposing the surface to a rinsing solution comprising 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium tetrafluoroborate, tetramethylammonium tetrafluoroborate, tetramethylammonium tetrafluoroborate, or hydrogen fluoride (HF).

In one embodiment, exposing the surface of the aluminum feature to fluorine without exposing the aluminum feature to nitrogen plasma or ammonium dip.

In one embodiment, a forming method of forming a bonded structure is disclosed. The forming method includes providing the element formed using the method, providing a second element having a second nonconductive field region and a conductive feature, directly bonding the nonconductive field region and the second nonconductive field region without an intervening adhesive, and directly bonding the aluminum feature and the conductive feature without an intervening adhesive.

In one aspect, a direct hybrid bonded structure is disclosed. The bonded structure can include a first element having a first nonconductive field region and a first aluminum feature. A surface of the first nonconductive field region and a surface of the first aluminum feature at least partially define a bonding surface of the first element. The bonded structure can include a second element having a second nonconductive field region and a second aluminum feature. A surface of the second nonconductive field region is directly bonded to the first nonconductive field region without an intervening adhesive along a bond interface and a surface of the second aluminum feature is directly bonded to the second aluminum feature without an intervening adhesive along the bond interface. The surface of the first aluminum feature includes a higher fluorine content than a portion of the first aluminum feature further away from the surface.

In one embodiment, the bond interface between the first and second aluminum features include one or multiple fluorine peaks.

In one embodiment, the first aluminum feature includes a first gradient of fluorine concentration decreasing away from the bond interface. The second aluminum feature can include a second gradient of fluorine concentration decreasing away from the bond interface.

In one embodiment, the first aluminum feature includes a first portion and a second portion over the first portion and at least partially defining the surface of the first aluminum feature. The second portion can include an average grain size smaller than an average grain size of the first portion. Most of the grains in the second portion can have a width in a range of 10 nm to 500 nm. The first aluminum feature can further include a barrier layer between the first and second portions.

In one embodiment, the bond interface between the first and second aluminum features includes less than 1000 ppm of oxygen.

In one embodiment, the bond interface between the first and second aluminum features includes less than 100 ppm of nitrogen.

In one embodiment, the first aluminum feature has a thickness in a range of 0.5 μm to 5 μm.

In one aspect, an element having a bonding surface configured to directly hybrid bond to another element is disclosed. The element can include a nonconductive field region, and an aluminum feature. A surface of the nonconductive field region and a surface of the aluminum feature together define the bonding surface of the element. The surface of the aluminum feature includes aluminum and fluorine.

In one embodiment, the aluminum feature has a thickness in a range of about 0.5 μm to 5 μm. The thickness of the aluminum feature can be in a range of about 1 μm to 3 μm.

In one embodiment, the aluminum feature includes a first portion and a second portion over the first portion. The second portion can define the surface of the aluminum feature. The second portion can include an average grain size smaller than an average grain size of the first portion. Most of the grains in the second portion can have a width in a range of 10 nm to 500 nm. A thickness of the second portion can be in a range of 0.1 μm to 0.3 μm.

In one embodiment, the surface of the aluminum feature is recessed from the surface of the nonconductive field region by about 2 nm to 20 nm.

In one aspect, a bonded structure is disclosed. The bonded structure can include a first element having a first nonconductive field region and a first conductive feature. A surface of the first nonconductive field region and a surface of the first conductive feature at least partially define a bonding surface of the first element. The first conductive feature includes a first portion and a second portion over the first portion and at least partially defines the surface of the first conductive feature. The first portion includes aluminum. The first conductive feature has a continuous sidewall along the first portion and the second portion. The second portion includes different metal composition from the first portion or includes fluorine at the surface of the first conductive feature. The bonded structure can include a second element having a second nonconductive field region and a second conductive feature. A surface of the second nonconductive field region that is directly bonded to the first nonconductive field region without an intervening adhesive along a bond interface and a surface of the second conductive feature that is directly bonded to the second conductive feature without an intervening adhesive along the bond interface.

In one embodiment, the second portion has an average grain size that is smaller than an average grain size of the second portion. Most of grains in the second portion can have a width in a range of 10 nm to 500 nm.

In one embodiment, the aluminum feature has a thickness in a range of 0.5 μm to 5 μm, and the second portion has a thickness in a range of 0.1 μm to 0.3 μm. The first and second portions can have different metal structures. The second portion can include aluminum. The surface of the conductive feature can include aluminum fluoride. The second portion can include copper.

In one aspect, an element having a bonding surface configured to directly bond to another element is disclosed. The element can include a nonconductive field region and a conductive feature that includes a first portion and a second portion over the first portion and at least partially defines a surface of the conductive feature. The first portion includes aluminum. The conductive feature has a continuous sidewall along the first portion and the second portion. The second portion includes different metal composition from the first portion or includes fluorine at the surface of the first conductive feature. A surface of the nonconductive field region and a surface of the aluminum feature together define the bonding surface of the element.

In one embodiment, the aluminum feature has a thickness in a range of about 0.5 μm to 5 μm, and the second portion has a thickness in a range of about 0.1 μm to 0.3 μm. The second portion can include aluminum. The surface of the conductive feature can include aluminum fluoride. The second portion can include copper.

A method of forming an element having a bonding surface configured to directly bond to another element is disclosed. The method can include forming a nonconductive field region and a first portion of a conductive feature. The first portion comprising aluminum. The method can include forming a second portion of the conductive feature over the first portion. The second portion at least partially defines a surface of the conductive feature. The first and second portions are defined by a single masking process. The second portion includes a different metal composition from the first portion or includes fluorine at the surface of the first conductive feature. A surface of the nonconductive field region and a surface of the conductive feature together define the bonding surface of the element.

In one embodiment, the second portion includes fine grain aluminum.

In one embodiment, the second portion includes copper.

In one embodiment, the second portion includes aluminum and fluorine.

In one embodiment, the method further includes exposing the surface of the conductive feature to fluorine. Exposing the surface of the conductive feature to fluorine can suppress aluminum oxide formation. Exposing the surface of the aluminum feature to fluorine can be conducted without exposing the aluminum feature to nitrogen plasma or ammonium dip.

Unless the context clearly requires otherwise, throughout the description and the claims, the words “comprise,” “comprising,” “include,” “including” and the like are to be construed in an inclusive sense, as opposed to an exclusive or exhaustive sense; that is to say, in the sense of “including, but not limited to.” The word “coupled”, as generally used herein, refers to two or more elements that may be either directly connected, or connected by way of one or more intermediate elements. Likewise, the word “connected”, as generally used herein, refers to two or more elements that may be either directly connected, or connected by way of one or more intermediate elements. Additionally, the words “herein,” “above,” “below,” and words of similar import, when used in this application, shall refer to this application as a whole and not to any particular portions of this application. Where the context permits, words in the above Detailed Description using the singular or plural number may also include the plural or singular number respectively. The word “or” in reference to a list of two or more items, that word covers all of the following interpretations of the word: any of the items in the list, all of the items in the list, and any combination of the items in the list.

Moreover, conditional language used herein, such as, among others, “can,” “could,” “might,” “may,” “e.g.,” “for example,” “such as” and the like, unless specifically stated otherwise, or otherwise understood within the context as used, is generally intended to convey that certain embodiments include, while other embodiments do not include, certain features, elements and/or states. Thus, such conditional language is not generally intended to imply that features, elements and/or states are in any way required for one or more embodiments.

While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the disclosure. For example, while illustrated embodiments include preparation for direct hybrid bonding, the skilled artisan will appreciate that the techniques taught herein can be useful for direct metal bonding even in the absence of direct dielectric bonding. Indeed, the novel apparatus, methods, and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the disclosure. For example, while blocks are presented in a given arrangement, alternative embodiments may perform similar functionalities with different components and/or circuit topologies, and some blocks may be deleted, moved, added, subdivided, combined, and/or modified. Each of these blocks may be implemented in a variety of different ways. Any suitable combination of the elements and acts of the various embodiments described above can be combined to provide further embodiments. It is also contemplated that various combinations or sub-combinations of the specific features and aspects of the embodiments may be made and still fall within the scope. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the disclosure.

Claims

What is claimed is:

1. A method of forming a bonding surface for direct hybrid bonding, the method comprising:

providing an element having a nonconductive field region and an aluminum feature; and

exposing a surface of the aluminum feature to fluorine, wherein the surface of the aluminum feature and a surface of the nonconductive field region at least partially define the direct bonding surface, wherein exposing the surface of the aluminum feature to fluorine comprises forming aluminum fluoride, aluminum fluoride oxide, or aluminum fluoride boron oxide.

2. The method of claim 1, wherein exposing the surface of the aluminum feature to fluorine suppresses aluminum oxide formation.

3. The method of claim 1, further comprising:

providing an aluminum layer over a back-end-of-line (BEOL) layer;

removing at least a portion of the aluminum layer to define the aluminum feature; and

providing a dielectric material proximate to the aluminum feature to define the nonconductive field region.

4. The method of claim 1, wherein the aluminum feature comprises a first portion and a second portion over the first portion and at least partially defining the surface of the aluminum feature, the second portion comprises an average grain size smaller than an average grain size of the first portion.

5. The method of claim 4, further comprising:

removing metal from an initial aluminum feature to leave the first portion below a recess of about 0.1 μm to 0.3 μm relative to the surface of the nonconductive field region; and

depositing the second portion into the recess over the first portion, the second portion having a microstructure different from the first portion.

6. The method of claim 5, wherein the second portion is deposited at a deposition temperature below about 100° C.

7. The method of claim 4, wherein the aluminum feature has a continuous sidewall along a sidewall of the first portion and a sidewall of the second portion.

8. The method of claim 1, wherein a thickness of the aluminum feature is in a range of about 0.5 μm to 7 μm.

9. The method of claim 1, wherein exposing the surface of the aluminum feature to fluorine comprises exposing the surface of the aluminum feature to a rinsing solution comprising 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium tetrafluoroborate, tetramethylammonium tetrafluoroborate, tetramethylammonium tetrafluoroborate, or hydrogen fluoride (HF).

10. A forming method of forming a bonded structure, the method comprising:

providing the element formed using the method of claim 1;

providing a second element having a second nonconductive field region and a conductive feature;

directly bonding the nonconductive field region and the second nonconductive field region without an intervening adhesive; and

directly bonding the aluminum feature and the conductive feature without an intervening adhesive.

11. A direct hybrid bonded structure comprising:

a first element having a first nonconductive field region and a first aluminum feature, a surface of the first nonconductive field region and a surface of the first aluminum feature at least partially defining a bonding surface of the first element; and

a second element having a second nonconductive field region and a second aluminum feature, a surface of the second nonconductive field region directly bonded to the first nonconductive field region without an intervening adhesive along a bond interface and a surface of the second aluminum feature directly bonded to the first aluminum feature without an intervening adhesive along the bond interface,

wherein the surface of the first aluminum feature comprises a higher fluorine content than a portion of the first aluminum feature further away from the surface.

12. The bonded structure of claim 11, wherein the bond interface between the first and second aluminum features comprise one or multiple fluorine peaks.

13. The bonded structure of claim 11, wherein the first aluminum feature comprises a first portion and a second portion over the first portion and at least partially defining the surface of the first aluminum feature, the second portion comprises an average grain size smaller than an average grain size of the first portion.

14. The bonded structure of claim 13, wherein most of the grains in the second portion have a width in a range of 10 nm to 500 nm.

15. The bonded structure of claim 11, wherein the bond interface between the first and second aluminum features comprises less than 1000 ppm of oxygen.

16. An element having a bonding surface configured to directly hybrid bond to another element, the element comprising:

a nonconductive field region; and

an aluminum feature, a surface of the nonconductive field region and a surface of the aluminum feature together at least partially defining the bonding surface of the element, the surface of the aluminum feature comprising aluminum and fluorine.

17. The element of claim 16, wherein the aluminum feature has a thickness in a range of about 0.5 μm to 5 μm.

18. The element of claim 16, wherein the aluminum feature comprises a first portion and a second portion over the first portion, the second portion defining the surface of the aluminum feature, the second portion comprises an average grain size smaller than an average grain size of the first portion, wherein most of the grains in the second portion have a width in a range of 10 nm to 500 nm.

19. The element of claim 18, wherein a thickness of the second portion is in a range of 0.1 μm to 0.3 μm.

20. The method of claim 2, wherein most of the grains in the second portion have a width in a range of 10 nm to 500 nm.

21. The method of claim 8, wherein the thickness of the aluminum feature is in a range of about 1 μm to 5 μm.

22. The method of claim 1, wherein the element includes an aluminum interconnect structure electrically connected to the aluminum feature.

23. The method of claim 1, wherein exposing the surface of the aluminum feature to fluorine comprises forming at least a portion that is free from aluminum oxide.

24. The method of claim 1, wherein exposing the surface of the aluminum feature to fluorine is conducted without exposing the aluminum feature to nitrogen plasma or ammonium dip.

25. The bonded structure of claim 11, wherein the first aluminum feature comprises a first gradient of fluorine concentration decreasing away from the bond interface.

26. The bonded structure of claim 25, wherein the second aluminum feature comprises a second gradient of fluorine concentration decreasing away from the bond interface.

27. The bonded structure of claim 13, wherein the first aluminum feature further comprises a barrier layer between the first and second portions.

28. The bonded structure of claim 11, wherein the bond interface between the first and second aluminum features comprises less than 100 ppm of nitrogen.

29. The bonded structure of claim 11, wherein the first aluminum feature has a thickness in a range of 0.5 μm to 5 μm.

30. The element of claim 29, wherein the thickness of the aluminum feature is in a range of about 1 μm to 3 μm.

31. The element of claim 16, wherein the surface of the aluminum feature is recessed from the surface of the nonconductive field region by about 2 nm to 20 nm.

32. A bonded structure comprising:

a first element having a first nonconductive field region and a first conductive feature, a surface of the first nonconductive field region and a surface of the first conductive feature at least partially defining a bonding surface of the first element, the first conductive feature including a first portion and a second portion over the first portion and at least partially defining the surface of the first conductive feature, the first portion comprising aluminum, the first conductive feature having a continuous sidewall along the first portion and the second portion, the second portion comprising different metal composition from the first portion or comprising fluorine at the surface of the first conductive feature; and

a second element having a second nonconductive field region and a second conductive feature, a surface of the second nonconductive field region directly bonded to the first nonconductive field region without an intervening adhesive along a bond interface and a surface of the second conductive feature directly bonded to the first conductive feature without an intervening adhesive along the bond interface.

33. The bonded structure of claim 32, wherein the second portion has an average grain size that is smaller than an average grain size of the first portion, most of grains in the second portion have a width in a range of 10 nm to 500 nm.

34. The bonded structure of claim 32, wherein the aluminum feature has a thickness in a range of 0.5 μm to 5 μm, and the second portion has a thickness in a range of 0.1 μm to 0.3 μm.

35. The bonded structure of claim 34, wherein the first and second portions have different metal structures.

36. The bonded structure of claim 34, wherein the second portion comprises aluminum.

37. The bonded structure of claim 36, wherein the surface of the conductive feature comprises aluminum fluoride.

38. The bonded structure of claim 34, wherein the second portion comprises copper.

39. An element having a bonding surface configured to directly bond to another element, the element comprising:

a nonconductive field region; and

a conductive feature including a first portion and a second portion over the first portion and at least partially defining a surface of the conductive feature, the first portion comprising aluminum, the conductive feature having a continuous sidewall along the first portion and the second portion, the second portion comprising different metal composition from the first portion or comprising fluorine at the surface of the first conductive feature, a surface of the nonconductive field region and a surface of the aluminum feature together at least paritally defining the bonding surface of the element.

40. The element of claim 39, wherein the second portion has an average grain size that is smaller than an average grain size of the second portion, most of grains in the second portion have a width in a range of 10 nm to 500 nm.

41. The element of claim 39, wherein the aluminum feature has a thickness in a range of about 0.5 μm to 5 μm, and the second portion has a thickness in a range of about 0.1 μm to 0.3 μm.

42. The element of claim 41, wherein the second portion comprises aluminum.

43. The element of claim 42, wherein the surface of the conductive feature comprises aluminum fluoride.

44. The element of claim 41, wherein the second portion comprises copper.

45. A method of forming an element having a bonding surface configured to directly bond to another element, the method comprising:

forming a nonconductive field region and a first portion of a conductive feature, the first portion comprising aluminum; and

forming a second portion of the conductive feature over the first portion, the second portion at least partially defining a surface of the conductive feature,

wherein the first and second portions are defined by a single masking process,

the second portion comprises a different metal composition from the first portion or comprises fluorine at the surface of the conductive feature, and

a surface of the nonconductive field region and a surface of the conductive feature together at least partially define the bonding surface of the element.

46. The method of claim 45, wherein the second portion comprises fine grain aluminum.

47. The method of claim 45, wherein the second portion comprises copper.

48. The method of claim 45, wherein the second portion comprises aluminum and fluorine.

49. The method of claim 45, further comprising exposing the surface of the conductive feature to fluorine.

50. The method of claim 49, wherein exposing the surface of the conductive feature to fluorine suppresses aluminum oxide formation.

51. The method of claim 49, wherein exposing the surface of the conductive feature to fluorine is conducted without exposing the conductive feature to nitrogen plasma or ammonium dip.